Steerable rotary drill bit assembly with pilot bit

ABSTRACT

A rotary drill bit assembly suitable for directionally drilling a borehole into an underground formation, the drill bit assembly having a bit body extending along a central longitudinal bit-body axis, and having a bit-body face at its front end, wherein an annular portion of the bit-body face is provided with one or more chip-making elements; a pilot bit extending along a central longitudinal pilot-bit axis, the pilot bit being partly arranged within the bit body and projecting out of the central portion of the bit-body face, the pilot bit having a pilot-bit face provided with one or more chip-making elements at its front end; a joint means arranged to pivotably connect the pilot bit to the bit body so that the bit-body axis and the pilot-bit axis can form a variable diversion angle; and a steering means arranged to pivot the pilot bit in order to steer the direction of drilling.

PROITITY CLAIM

The present application claims priority on European Patent Application01306106.4 filed on 16 Jul. 2001.

FIELD OF THE INVENTION

The present invention relates to a rotary drill bit assembly, which issuitable for directionally drilling a borehole into an undergroundformation.

BACKGROUND OF THE INVENTION

In modern drilling operations, for example when drilling a wellbore inan oil or gas field, it is often desired to change the direction in thecourse of drilling. Generally one wishes to deviate the direction intowhich the drill bit at the lower end of a drill string progresses., awayfrom the central longitudinal axis of the lower part of the drillstring. Several drilling systems and methods have been developed forthis purpose in the past.

U.S. Pat. No. 4,836,301 discloses a system and method for directionaldrilling. In the known system the drill bit is connected via a universalpivoting mechanism to the lower end of the drill string. The drill bitcan be tilted so that the longitudinal axis of the drill bit can form asmall deviation angle with the axis of the lower part of the drillstring. The known system further comprises a steering means for rotatingthe drill bit in an orbital mode with respect to the lower part of thedrill string. The steering means thereto comprises a flow deflector forproviding hydrodynamical force in order to rotate the tilted drill bitazimuthally with respect to the lower part of the drill string asneeded.

During normal operation of the known system, the drill string with thedrill bit at its end is set to rotate, and the drill bit is tilted andcounter-rotated in an orbital mode relative to the lower part of thedrill string such that the axis of the drill bit remains geostationary.

The known system has the disadvantage that it requires large tiltingforces on the bit, and that a complex but robust mechanism is needed forthe universal pivoting mechanism in order to withstand the tilting anddrilling forces at the same time.

Other systems known in the art are based on bending the lower part ofthe drill string above the drill bit, or on pushing the drill bit intothe desired direction by applying side forces to the shaft of the drillbit.

These other systems also require complex and robust mechanisms in orderto provide the large tilting forces to the bit.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved drillbit and drill bit assembly suitable for directional drilling of aborehole, which is mechanically simpler than the known systems.

It is a further object to provide an improved method for directionaldrilling of a borehole.

To this end the present invention provides a rotary drill bit assemblysuitable for directionally drilling a borehole into an undergroundformation, the drill bit assembly comprising a bit body extending alonga central, longitudinal bit-body axis, the bit body having a bit-bodyface at its front end and being attachable to a drill string at itsopposite end, wherein an annular portion of the bit-body face isprovided with one or more chip-making elements; a pilot bit extendingalong a central longitudinal pilot-bit axis, the pilot bit being partlyarranged within the bit body and projecting out of the central portionof the bit-body face, the pilot bit having a pilot-bit face at its frontend provided with one or more chip-making elements; a joint meansarranged to pivotably connect the pilot bit to the bit body so that thebit-body axis and the pilot-bit axis can form a variable diversionangle; and a steering means arranged to pivot the pilot bit in order tosteer, during normal operation, the direction of drilling.

The bit body, pilot bit and joint means are comprised in a drill bitaccording to the invention.

There is further provided a method for directional drilling of aborehole into an underground earth formation, comprising the steps of

providing a rotary drill bit attached to the lower end of a drillstring, the rotary drill bit comprising a bit body extending along abit-body axis coaxial with the lower part of the drill string, andhaving a bit-body face at its front end, wherein an annular portion ofthe bit-body face is provided with one or more chip-making elements, and

a pilot bit extending along a pilot-bit axis and projecting out of thecentral portion of the bit-body face, the pilot bit having a pilot-bitface at its front end provided with one or more chip-making elements;which pilot bit is pivotably arranged with respect to the bit body sothat the bit-body axis and the pilot-bit axis can form a certaindiversion angle;

setting the pilot bit along the pilot-bit axis at a selected diversionangle with respect to the bit-body axis;

providing at the same time drilling torque around the pilot-bit axis tothe pilot bit and drilling torque around the bit-body axis to the bitbody, and

wherein the orientation of the pilot-bit axis in space is keptsubstantially constant during at least one revolution of the bit bodyabout the bit-body axis.

With the pivotable pilot bit having its face some distance ahead of theface of the bit body, a tilted pilot borehole section can be drilled,wherein the depth is approximately equal to the distance betweenpilot-bit face and bit-body face. Due to the smaller size of the pilotbit, a smaller tilting force is needed for the pilot bit as compared totilting the whole drill bit directly. The pilot borehole section servesas a guide for the cutting action of the bit body. The pilot bit in thepilot borehole section exerts a guiding force on the bit body, andthereby guides or levers the bit body including the attached drillstring into the desired direction. The guiding force on the bit bodyacts near the bit-body face, thereby rather pulling than pushing the bitbody into the desired direction, which is a fundamental difference tothe directional drilling systems and methods known in the art.

In general, drilling torque to the pilot bit can be providedindependently from the drilling torque provided from the drill string tothe bit body. Suitably, the pilot bit is driven by the drilling torqueprovided by the drill string. In this case, if a straight borehole is tobe drilled no steering is needed, and the drill bit can perform similarto a conventional rotary drill bit. The joint means can suitably bearranged so as to transmit drilling torque from the drill string, whichis fixedly connected to the bit body, to the pilot bit. Preferably, thejoint means torque-locks the pilot bit to the bit body, so that onerevolution of the bit body about the bit-body axis results in onerevolution of the pilot bit about the pilot-bit axis. It will beunderstood, however, that a gearing mechanism can be arranged so thatthe pilot bit rotates with a different angular speed than the bit body.The pilot bit can also be driven from a different source not directlycoupled to the rotary action of the drill string, such as a mud motor.

In the case that the pilot bit and bit body are rotated together, eachabout its respective longitudinal axis, the pilot bit is suitablypivoted such that the pilot-bit axis performs an orbital motion withrespect to the bit-body axis, in opposite direction and with the sameangular velocity of the rotation of the bit body. In this way thepilot-bit axis can be kept substantially stationary in space, withrespect to the non-rotating environment. In order to allow the orbitalmotion the joint means is a spherical joint means, which allows thepilot bit to rotate azimuthally about the bit-body axis while thepilot-bit axis is pivoted at a non-zero diversion angle.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE shows schematically an example of a rotary drill bit assembly1 for directionally drilling a borehole into an underground formation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT.

The invention will now be described in more detail with reference toFIG. 1.

FIG. 1 shows schematically an example of a rotary drill bit assembly 1for directionally drilling a borehole into an underground formation,according to the present invention. The drill bit assembly 1 comprises adrill bit 2 having a drill bit body 3, which is fixedly connected to thelower end of a tubular drill string 5. The bit body 3 extends from thedrill string 5 along a central longitudinal bit-body axis 8 and has abit-body face 10 at its front end. The bit-body face 10 is provided withchip-making elements in the form of polycrystalline diamond cutters 12,which are arranged around a central opening 14 in the bit-body face 10and thereby forming an annular portion of the bit-body face 10. Thecutters are suitably designed to give ease of side cutting.

The bit body 3 is provided with a central longitudinal passageway 16providing fluid communication between the interior of the drill string 5and the opening 14 of the bit body 3. The passageway 16 at the side ofthe opening 14 is provided with a sleeve 18, which is connected to thebit body 3. Further, fluid nozzles 19 are provided, which are in fluidcommunication with the passageway 16.

The drill bit 2 further comprises a pilot bit 20, which is partlyarranged within the bit body 3 and projects out of the central portion14 of the bit-body face 10. At its front end the pilot bit 20 has apilot-bit face 25, which is provided with chip-making elements in theform of polycrystalline diamond cutters 27. The pilot bit is alsoprovided with fluid nozzles 28, which are in fluid communication withthe passageway 16. The pilot bit 20 further has a gauge side 29.

The pilot bit 20 is connected to the bit body 3 through a sphericaljoint means arranged at the front end of the sleeve 18, and shownschematically at reference numeral 30. The spherical joint means 30allows pivoting of the pilot bit 20 with respect to the bit body 3, sothat the central longitudinal pilot-bit axis 32 and the bit-body axis 8can form a non-zero diversion angle. In the FIGURE the pilot bit ispivoted about an axis (not shown) perpendicular to the paper plane, andthe diversion angle is indicated by the symbol a. The spherical jointmeans 30 also allows rotation of the pilot bit 20 about the bit-bodyaxis 8 while the pilot-bit axis is pivoted by a non-zero diversionangle.

The spherical joint means 30 further is arranged so as to torque-lockthe pilot bit 20 to the bit body 3, so that one revolution of the bitbody 3 about the bit-body axis 8 results in one revolution of the pilotbit 20 about the pilot-bit axis 32.

The spherical joint means can suitably be designed based on a jointknown in the art as universal joint. Well-known types of universaljoints are for example Hooke, Bendix-Weiss, Rzeppa, Tracta, or doubleCardan joints. The advantage of the universal joint is that no separatedriving source and drill string for the pilot bit is needed, and thatthe pilot bit and the bit body rotate jointly with the same averageangular velocity so that abrasive forces at the joint means can be keptto a minimum.

The drill bit assembly 1 further comprises a steering means for steeringthe drill bit 2, which steering means is generally referred to byreference numeral 40. The steering means 40 is arranged to pivot thepilot bit 20 in order to steer the drill bit 2. To this end, thesteering means comprises a steering lever 42 extending from a contactarrangement 45 with the joint means 30 to a lever point 47 in thepassageway 16 of the bit body 3. The contact arrangement 45 and thelever point 47 are located along the pilot-bit axis 32. The contactmeans 45 has the form of a bearing (not shown), which allows rotation ofthe pilot bit 20 about the pilot-bit axis 32 relative to the steeringlever 42. By moving the lever point 47 the pilot bit can be pivoted, anddue to the contact means in form of a bearing the orientation of thepilot bit can be steered independently of the rotation of the pilot bit.

In order that the pilot bit 20 can drill into a certain direction, thesteering lever 42 needs to be oriented, and the lever point 47 issuitably set to remain geostationary during rotation of the bit body 3.Positioning is done using a positioning lever 52 of the steering means,which positioning lever 52 is connected at one end to the lever point47. For compensating the rotation of the bit body 3 a rotation means inthe form of step motor 55 is provided, which is connected to the otherend of the positioning lever 52. The housing of the step motor 55 isarranged in a fixed orientation with the drill string 5 and the bit body3. The lever point 47 can be kept at a geostationary location byrotating the positioning lever 52 relative to the bit body 3 about thebit-body axis 8, in opposite direction and with the same angularvelocity as the rotating bit body 3, and while keeping the offset of thelever point 47 from the bit-body axis 8 constant.

The steering means further comprises a directional sensor package 58 formeasuring data to determine the actual drilling trajectory of the drillbit; a surface communications package 60 including a mud pulser; and asteering control package 62 for controlling the positioning and rotationof the steering lever 42 in response to data from the directional sensorpackage 60, to data about the angular velocity of the drill string,and/or to commands received from the surface.

The sleeve 18 with the spherical joint means 30 and the attached pilotbit 20 forms a closure element for the passageway 16. As shown in FIG. 1this closure element prevents access from the interior of the drillstring 5 to the exterior of the bit body in the borehole via opening 14.The sleeve 18 can be removably attached to the bit body 3, for exampleby a latching mechanism (not shown), which is arranged so that theclosure element can be selectively connected to and disconnected fromthe bit body. When the closure element has been removed, the exterior ofthe bit body in the borehole can be accessed from inside the drillstring through the opening 14.

Normal operation of the embodiment shown in FIG. 1 will now bediscussed. If a straight wellbore is to be drilled, the pilot-bit axis32 is aligned with the bit-body axis 8, and to this end the lever point47 is moved to a location on the bit-body axis 8. By putting drillingtorque and weight on the drill bit 2, the pilot bit and bit body rotatejointly due to the torque lock of the spherical joint 30, and the drillbit will perform like a conventional drill bit of similar overallgeometry. In particular there is no need in this situation to rotate thesteering lever 42 by the step motor 55 relative to the bit body 3.

If then a curved wellbore is to be drilled, the pilot-bit axis 32 is setto deviate from the bit-body axis 8 by positioning the lever point 47away from the bit-body axis. To this end, the steering control packageappropriately steers the positioning lever 52, so that the steeringlever 42 has the desired orientation in space (diversion angle andazimuthal orientation). The diversion angle between bit-body axis andpilot-bit axis can for example be set between 1 and 5 degrees, butlarger or smaller values are also possible.

Drilling torque is provided to the bit body 3 and via the sphericaljoint means 30 at the same time to the pilot bit 20, so that the pilotbit progresses into the formation as guided by steering lever. The stepmotor 55 is activated to counteract the rotation of the bit body byrotating the positioning lever 52, so that the steering lever 42 remainssubstantially geostationary during at least one rotation of the bit body3. The pilot bit 20 forms a pilot borehole section that deviates fromthe bit-body axis 8, and the bit body 3 is consequently levered towardsthe direction of the pilot borehole section by a guiding force exertedby the pilot bit via the joint means. The gauge side 29 of the pilot bit20, which is subjected to abrasive forces from contact with theformation in the pilot borehole section, is suitably designed tominimize abrasion. The gauge side 29 can for example be manufacturedfrom diamond or can include PDC gauge protection elements.

The actual overall direction of drilling is monitored by the directionalsensor package 58. Data obtained from the directional sensor packageand/or commands received from the surface via the surface communicationspackage 60 are processed by the steering control package 62. Thesteering control package then controls the steering lever to match thedesired and actual drilling trajectories.

The direction of drilling can be controlled by varying the orientationof the pilot bit (steering lever) in space (magnitude of the diversionangle and azimuthal orientation), suitably on a time scale longer thanone revolution of the bit body. The steering means can be arranged toset the magnitude steplessly, or to switch between a predeterminednon-zero diversion angle and zero diversion angle. The predetermineddiversion angle can be a maximum diversion angle of the joint means.

While the illustrative embodiments of the invention have been describedwith particularity, it will be understood that various othermodifications will be readily apparent to, and can be easily made by oneskilled in the art without departing from the spirit of the invention.Accordingly, it is not intended that the scope of the following claimsbe limited to the examples and descriptions set forth herein but ratherthat the claims be construed as encompassing all features which would betreated as equivalents thereof by those skilled in the art to which thisinvention pertains.

1. A rotary drill bit assembly suitable for directionally drilling aborehole into an underground formation, the drill bit assemblycomprising: a bit body extending along a central longitudinal bit-bodyaxis, the bit body having a bit-body face at its front end and beingattachable to a drill string at its opposite end, wherein an annularportion of the bit-body face is provided with one or more chip-makingelements; a pilot bit extending along a central longitudinal pilot-bitaxis, the pilot bit being partly arranged within the bit body andprojecting out of the central portion of the bit-body face, the pilotbit having a pilot-bit face at its front end provided with one or morechip-making elements; a joint means arranged to pivotably connect thepilot bit to the bit body so that the bit-body axis and the pilot-bitaxis can form a variable diversion angle; and a steering means arrangedto pivot the pilot bit in order to steer, during normal operation, thedirection of drilling.
 2. The rotary drill bit assembly according toclaim 1, wherein the joint means is a spherical joint means.
 3. Therotary drill bit assembly according to claim 1, wherein the joint meansis further arranged to torque-lock the pilot bit to the bit body.
 4. Therotary drill bit assembly according to claim 1, wherein the steeringmeans, for drilling at a constant non-zero diversion angle, is arrangedso as to pivot the pilot bit with respect to the bit body such that theorientation of the pilot-bit axis in space remains substantiallyconstant during at least one revolution of the bit body about thebit-body axis.
 5. The rotary drill bit assembly according to claim 4,wherein the steering means comprises a steering lever extendingsubstantially along the pilot-bit axis from a contact arrangement withthe joint means to a lever point within the interior of the bit body,and wherein the pilot bit can be pivoted by changing the position of thelever point with respect to the bit body.
 6. The rotary drill bitassembly according to claim 5, wherein the steering means furthercomprises a rotation means connected to the bit body, and wherein thelever point is set to remain substantially at its point in space by therotation means which is arranged to rotate the lever point relative tothe bit body about the bit-body axis, at constant offset from thebit-body axis, in opposite direction and with the same angular velocityas the rotating bit body.
 7. The rotary drill bit assembly according toclaim 5, wherein the contact arrangement with the joint means comprisesa bearing arranged to allow rotation of the joint means about thepilot-bit axis relative to the steering lever.
 8. The rotary drill bitassembly according to claim 1, wherein the steering means furthercomprises a steering control means arranged to control the direction ofthe steering lever during normal operation.
 9. The rotary dill bitassembly according claim 8, wherein the steering control means comprisesone or more of: a directional sensor package, a surface communicationspackage, a rotation means for rotating the lever point about thebit-body axis.
 10. The rotary drill bit assembly according to claim 1,wherein the bit body is provided with a passageway providing fluidcommunication between the interior of an attached drill string and thewell bore exterior of the bit body, and with a removable closure elementarranged to selectively close the passageway, wherein the closureelement comprises the pilot drill bit.
 11. A rotary drill bit suitablefor directionally drilling a borehole into an underground formation, thedrill bit comprising a bit body extending along a central longitudinalbit-body axis, the bit body having a bit-body face at its front end andbeing attachable to a drill string at its opposite end, wherein anannular portion of the bit-body face is provided with one or morechip-making elements; a pilot bit extending along a central longitudinalpilot-bit axis, the pilot bit being partly arranged within the bit bodyand projecting out of the central portion of the bit-body face, thepilot bit having a pilot-bit face at its front end provided with one ormore chip-making elements; and a joint means arranged to pivotablyconnect the pilot bit to the bit body so that the bit-body axis and thepilot-bit axis can form a variable diversion angle.
 12. The rotary drillbit according to claim 11, wherein the joint means is a spherical jointmeans.
 13. The rotary drill bit according to claim 11, wherein the jointmeans is further arranged to torque-lock the pilot bit to the bit body.14. The rotary drill bit according to claim 11, wherein the bit body isprovided with a passageway providing fluid communication between theinterior of an attached drill string and the well bore exterior of thebit body, and with a removable closure element arranged to selectivelyclose the passageway, wherein the closure element comprises the pilotdrill bit.
 15. A method for directional drilling of a borehole into anunderground earth formation, comprising the steps of providing a rotarydrill bit attached to the lower end of a drill string, the rotary drillbit comprising a bit body extending along a bit-body axis coaxial withthe lower part of the drill string, and having a bit-body face at itsfront end, wherein an annular portion of the bit-body face is providedwith one or more chip-making elements, and a pilot bit extending along apilot-bit axis and projecting out of the central portion of the bit-bodyface, the pilot bit having a pilot-bit face at its front end providedwith one or more chip-making elements; which pilot bit is pivotablyarranged with respect to the bit body so that the bit-body axis and thepilot-bit axis can form a certain diversion angle; setting the pilot bitalong the pilot-bit axis at a selected variable diversion angle withrespect to the bit-body axis; providing at the same time drilling torquearound the pilot-bit axis to the pilot bit and drilling torque aroundthe bit-body axis to the bit body, and wherein the orientation of thepilot-bit axis in space is kept substantially constant during at leastone revolution of the bit body about the bit-body axis.
 16. The methodaccording to claim 15, wherein the pilot bit and bit body aretorque-locked.
 17. The method according to claim 15, wherein thediversion angle is steplessly varied, in order to drill into along acertain trajectory.
 18. The method according to claim 15, wherein thediversion angle is varied by switching between zero and a predeterminednon-zero diversion angle, in order to drill along a certain trajectory.